1. Field of the Invention
The invention relates to a remote temperature sensing device and a method of remote temperature sensing for a rotating item, in which a Curie magnetic transition of an amorphous ferromagnetic material is utilized. More particularly, the present invention provides a device and a method of remote temperature sensing of a rotating component of a moving machine.
2. Description of the Related Art
There are a number of techniques and tools available to measure temperature, including a well-known temperature indicator such as a classical mercury thermometer, a thermocouple, a resistance thermometer, a bi-metal, and the like. All of these utilize some basic physical phenomena which vary with temperature, thus each having a unique feature. For example, mercury thermometry is effective for visual sensing of temperature, but is not suited for direct conversion of a temperature to an electrical signal. A thermocouple which utilizes thermoelectric effects of metals is more suited if an electronic reading of a temperature is needed. However, a thermocouple has to be wired to a voltmeter, which converts an electrical signal to a corresponding temperature. A resistance thermometer, which utilizes the temperature dependence of resistivity of a metal, also has to be wired to a voltmeter. These techniques requiring wire connections between a sensor and a temperature indicator, therefore, are not suited for remote sensing of a temperature. In such cases, as in sensing the temperature of a moving tire, remote sensing of the temperature becomes necessary, and a temperature sensor utilizing the temperature dependence of resistivity of a semiconductor is used. However, this type of sensor requires a power source to transmit a signal. The sensor is installed on a rotating rim or tire. Accordingly, it is difficult to apply the power from the automobile body to rotating tires, and also it is necessary to utilize a battery in order that the temperature monitoring device can correctly function. This kind of a sensor must respond to the temperature and send a temperature-dependent signal wirelessly to a detector for further signal processing. This type of temperature sensing is increasingly needed for automotive tires to prevent the pneumatic tires from bursting due mainly to the temperature rise of the tires during operation.
One such sensor may be realized by utilizing the Curie magnetic transition in a ferromagnetic material such as iron, which has a ferromagnetic Curie temperature above which ferromagnetism disappears along with all related phenomena such as high magnetization and permeability. The change of the magnetization and the permeability of a ferromagnetic material at the Curie temperature may be readily detected remotely by conventional magnetometry. U.S. Pat. No. 4,052,696 discloses a tire temperature sensing circuit that utilizes the Curie magnetic transition in a ferrite element. The magnetic change at the Curie transition is detected by an inductive coupling effect. Thus, this technique requires a very small gap between the ferrite-based temperature sensor and a stationary detector to maintain a reliable detecting signal. The distance of this gap is thus very small because ferrites usually have relatively low magnetic permeabilities ranging from 80 to 2000, as is noted, for example, on page 498 of “Physics of Magnetism” by S. Chikazumi (John Wiley & Sons, NY, 1964). Thus, there is a need for temperature sensors which do not require batteries and are capable of remote detection within a practical detection range. Also needed is a temperature sensing device with as little electrical circuitry as possible.